The Gram-negative bacterium Francisella tularensis is the causative agent of the zoonotic disease tularemia. Humans acquire tularemia from contact with infected tissues or materials, insect bites, consumption of contaminated food or water, or inhalation of aerosols. F. tularensis consists of 3 subspecies—tularensis, holarctica and mediasiatica—which differ in their geographic distributions and in their virulence in humans. F. tularensis subspecies tularensis, found almost exclusively in North America is highly virulent for humans. As few as 10 organisms subcutaneously or 25 organisms by inhalation can lead to a severe infection. F. tularensis subspecies holarctica (found in North America and in Europe) and subspecies mediasiatica (found in Asia) are of lower virulence. Because of its high infectivity and capacity to cause severe morbidity and mortality, F. tularensis subspecies tularensis is considered a potential agent of bioterrorism.
A vaccine against F. tularensis was developed a half-century ago, but it has not been approved for general use. This vaccine, called Live Vaccine Strain, or LVS, is an attenuated form of Francisella tularensis subspecies holarctica, a much less virulent subspecies of F. tularensis than the highly virulent subspecies of concern as a bioterrorist agent, F. tularensis subspecies tularensis. The LVS vaccine is poorly characterized, unstable in that different colonial morphology types emerge on culture, and somewhat toxic to humans vaccinated with it. Moreover, it may not protect against the high doses of F. tularensis subspecies tularensis that might be released in an airborne bioterrorism attack.